1. Field of Use
This invention relates generally to planar integrated optics. More specifically, this invention concerns devices which couple light waves into planar integrated optic circuits.
2. Description of the Prior Art
The recent high interest in single mode integrated optic circuits has been motivated by recent developments in integrated optic gyro programs, chip-to-chip and board-to-board VLSI interconnects, LAN/WAN communication, optical storage (disk) medias and other related technologies. Of particular current interest is the integrated optics imaging systems such as an integrated optical head for optical disk applications used in the well familiar compact disks (CD). Other important uses of the day include laser printing devices and medical devices, both of which have greatest utility when made compact in size. The planar topology of these devices is desirable and superior because it is close to PC board geometry. A third use of special interest is flat displays in compact electro-optic systems.
Planar integrated optics differs from predecessor freespace devices in that the light waves which propagate in these devices move in a planar light wave medium which is highly dispersive. The predecessor to planar integrated optics might be called bulk optics in which the system comprises a discrete light source, lens system and image plane, somewhat analogous to the source, lens, and image setup of a camera. The medium of propagation in the bulk optics type system is generally free space and therefore has little or no dispersion, i.e., the index of refraction n is approximately equal to the value 1 for all wavelengths (.lambda.). As a consequence, bulk optics systems can uniformly operate in a multi-mode continuous status in which incident light is accepted by the system at all angles of incidence and thus contain an infinite number of modes or distinct light waves traveling in it.
In planar integrated optics, on the other hand, the index of refraction n.sub.e is strongly dependent on wavelength and, furthermore, depends on the mode of the light wave. Propagation of light waves in planar integrated optics systems therefore is usually, necessarily discrete mode. That is, only discrete sets of angles of incidence are accepted by the system and only discrete sets of modes are present in the system. This type of system is called multi-mode discrete and is characterized by a planar path having a thickness of approximately 10.lambda.. Discrete multi-mode systems typically suffer from problems such as intermode fluctuations, however, and thus single mode systems, where waveguide thickness is approximately equal to one wavelength, and continuous multi-mode systems, where waveguide thickness is much greater than one wavelength, are regularly used. Single-mode is usually better suited for planar applications than multi-mode.
Unlike bulk optics, planar integrated optics is thin. Cross-sectional sizes of waveguides involved are on the order of one wavelength. Therefore, contrary to the continuous-mode propagation structure of bulk optics, propagation in planar systems is discrete-mode.
Certain problems, however, remain in state-of-the-art planar integrated optics systems. Planar integrated optics generally require light sources having high wavelength tolerance specifications. Light waves emanating from light sources having low wavelength tolerance specifications are often not coupled into the system at all. LED light sources generally have line widths of fifty nanometers and significant variations in the position of the center wavelength. Laser diodes (LD), however, have far narrower line widths, on the order of 1.ANG., and have a temperature drift ratio of about 1.ANG. per 1.degree. C. Currently mass produced LD's for CD's where the center line width is about 780 nanometers are quite wavelength-variable. Variations of 10 to 20 nanometers in the position of the center wavelength among LD's is not uncommon. These variations in light sources are satisfactory for current CD technology and consequently there is little market demand for low cost LD's having controllable bandwidths such that wavelength shifts are less than 1 nm. These LD's are unsatisfactory, however, for more precise planar single-mode integrated optics use.
The reason that current LD's are unsatisfactory for single-mode planar integrated optics use is because such planar systems are highly dispersive. Dispersion causes the refractive index to change as a function of wavelength resulting in each wavelength light wave propagating through the waveguide in a different manner. For a general discussion of planar waveguide propagation see T. Jannson, Information Capacity of Bragg Holograms in Planar Optics, 71 J. Opt. Soc. of Am. (JOSA) 342, 346 (1981); T. Jannson & J. Sochacki, Primary Aberrations of Thin Planar Surface Lenses, 70 JOSA 1079, 1080 (1980) incorporated herein by reference.
State-of-the-art couplers do not deal well with such wavelength variation. Three basic types of couplers have been used in planar integrated optic systems to couple the free space propagating light wave into the planar waveguide. The three types of couplers that have been used are surface relief grating couplers, prism couplers, and direct coupling. Prism couplers are discussed in detail in R. Ulrich, Optimum Excitation of Optical Surface Waves, 61 JOSA 1467 (1971); R. Ulrich, Theory of the Prism-Film Coupler by Plane-wave Analysis, 60 JOSA 1337 (1970); and P. Tien and R. Ulrich, Theory of Prism-FIlm Couplers and Thin-Film Light Guides 60 JOSA 1325 (1970) incorporated by reference herein. All of these types of couplers are discussed generally in J. Jannson, Ph.D., Dissertation, The University of New Mexico, Albuquerque, New Mexico, May 1984, incorporated by reference herein. With respect to grating couplers in particular see R. Ulrich, Efficiency of Optical-Grating Couplers, 63 JOSA 1419 (1973) incorporated by reference herein. Direct coupling is satisfactory in most cases but not for use in integrated optical heads where the light source is located at the top of the plane of the recorded surface. The prism and grating couplers operate in the state-of-the-art by coupling the light wave through the prism or grating and directly into the single-mode wave guide. These types of couplers, although in widespread use, are of very narrow band, as described above, thus making them susceptible to and inefficient in the presence of light source wavelength shift.
One type of state of the art coupler that attempts to circumvent the narrow band problem mechanically varies the coupler structure in order to present a different angle of incidence for each wavelength. These types of couplers are highly impractical for planar integrated optics use. Their usefulness is, therefore, markedly reduced. Narrow band couplers comprising the state of the art have limited the large potential of planar integrated optics.